A posse of Britain’s most powerful businessmen walked through the door of 10 Downing Street 10 days ago to deliver a warning to the prime minister.

The group, which included Stephen Green of HSBC, Vodafone’s Arun Sarin, and Andrew Witty of Glaxo Smith Kline, were calling on behalf of the Multinational Chairmen’s Group, a little-known but influential body that represents the upper echelons of British business.

They were following a well-worn trail. A week earlier, the City’s top bankers had trooped through that famous front door to thrash out the government’s response to the credit crisis.

This time, the chief subject was not bank liquidity, but tax. The group had an alarming message for Gordon Brown and chancellor Alistair Darling, alerting them to yet another potential slip-up in what has proved the government’s most accident-prone field.

Britain was at risk of a corp-orate exodus, they said. Treasury proposals to change the treatment of foreign earnings had given a hard edge to general unrest about corporate tax.

If Brown and Darling did not get a grip, multinationals would up sticks and shift their tax bases offshore. The UK would lose an estimated £1 billion a year in tax revenue, and there would be an ebbing away of corporate influence and prestige, which would slowly strip Britain of its unofficial title as the headquarters of world business.

There was even a live example to throw at Brown and Darling. Only a week earlier, Shire Pharmaceuticals, an FTSE 100 drugs company, announced, to the surprise of most in the City, that it was quitting Britain. It would put in place a new corp-orate structure, with a holding group at the top of the tree, incorporated in Jersey and paying tax in Ireland. New Shire would replace old Shire in seven weeks.

Since the Shire bombshell, more companies have come out of the woodwork. United Business Media, the media, conferences and exhibitions group, said it, too, was off to Ireland, while several others — Astra Zeneca, Glaxo Smith Kline, International Power, WPP, Aegis and ITV — confirmed that they were examining their options.

In Downing Street, Brown and Darling appeared surprised by the reaction to their tax proposals, according to a senior tax laywer familiar with the meeting. and nonplussed by the strength of feeling on the issue — feeling that was most forcefully expressed by Vodafone’s Sarin.

“They were only just waking up to what it really meant,” the lawyer said.

Labour has had a string of fiscal debacles — a change in capital-gains tax, aimed at private equity but catching small businesses and entrepreneurs; alterations to the non-domicile regime that alienated the City; and, most damaging of all, the elimination of the 10p tax band that nearly triggered a Commons revolt by Labour MPs. Coming on top of all this, the corporate-tax unrest must have been an unwelcome bolt from the blue.

Brown and Darling wasted no time. On Monday, Darling made a speech at Chatham House in which he promised to set up a task force that would conduct a wide-ranging review of Britain’s corporate-tax regime.

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